Our recent studies have concerned the localization and function of calcium ions at the cell surface, a problem that is basic to many biological phenomena. Specific questions that interest us are the interaction of calcium with components of the cell membrane (lipids, carbohydrates, proteins, ATPases), the mechanism of calcium transport across epithelia, cellular aspects of calcification, and intracellular calcium homeostasis. Our approach takes advantage of recent advances in the cellular localization of calcium. It has been found that under appropriate conditions of fixation and processing of tissues, calcium can be retained within tissues and is sufficiently electron-opaque to be visualized in the electron microscope. New techniques of microscope analysis, including use of frozen unfixed material. When correlated with physiological and morphological studies, should make possible significant advances in diverse aspects of the biology of calcium.